(a) Field of the Invention
This invention is broadly concerned with a two component type dry developer for use in dichromatic electrophotography and particularly concerned with two kinds of developers wherein there exists a specific relation between toners and carriers therefor.
(b) Description of the Prior Art
Conventionally, the cycling process comprising electrification-exposure-development-transfer has been repeated in order to obtain a dichromatic image by using an electrophotographic photosensitive element with the undesirable result that a shear in the image is caused and accordingly a clear-cut copy is not obtainable.
In U.S. Pat. No. 3,236,639 there is further proposed a three component type developer comprising mixing with carrier particles, two kinds of toner particles, which are different in color from each other and are triboelectrifiable with a polarity opposite to each other. However, such a three component developer is defective in that said developer is apt to bear a mixed color because it is originally used in a combined manner, thereby hampering the production of a clear-cut dichromatic image, and further such a developer is unable to exhibit a satisfactory developing stability because the friction stability of the toner used is inferior.
To cope with this, public attention is now focused on a process for the production of a dichromatic image comprising the steps of forming a both positively and negatively electrified electrostatic latent image on one photosensitive element, developing said latent image successively with separate developers which are different in charged polarity from each other (namely, two kinds of developers which are different in charged polarity from each other), and then transferring the resulting toner image onto a common paper or the like.
Even the so-called two component type developing system for use in dichromatic electrophotography, which comprises developing an electrostatic latent image formed by means of this positive and negative electrification successively with toners which are different in polarity and color from each other, however, is noted to suffer from the problem that occasionally the toner provided on the photosensitive element by an earlier development is scraped off upon effecting a subsequent development. The occurrence of this scraping-off phenomenon not only deteriorates the density of the image tinted with the earlier developed color but also induces the toner thus scraped off to mingle in the developer used in subsequent development, which latter developer contains toner tinted with a different color than the first-used developer, whereby the electrified state of the toner contained in this developer (namely, the developer used in the subsequent development) is disturbed and ultimately the image quality is widely deteriorated thereby.